Add a
banner
to the skyscraper category (create a new one if you want)
Create a new billboard - making sure that the ad(s) in the billboard only select those from the skyscraper ad.
Edit the forumdisplay script. Untick the 'use master'. Assign your billboard created in the previous step to an availible $GAB_ads[x].
Put the $GAB_ads[x] in one of your templates

HTHs

GAB

GAB is a highly configurable banner system that allows you to customize banners for vBulletin easily. Determine which usergroups see banners, which ones can disable them, which pages have banners and where. GAB is the one stop shop for banner advertising.